1. Field of the Invention
The present invention relates to high-voltage bidirectional switches controllable by a low-voltage signal. The present invention more specifically relates to bidirectional switches connected in series with a load supplied by a high A.C. voltage, for example the mains and, among such switches, those that are controllable to be turned off and on.
2. Discussion of the Related Art
High-voltage switches for an A.C. load of the type to which the present invention applies are used, in particular, each time an interface circuit between a low-voltage control system, for example a microcontroller, and a load to be supplied by an A.C. high voltage, for example, an engine, a heating resistor, etc., is required.
An example of application of the present invention relates to electric household appliances, for example washing machines, in which several loads (engines, pumps, heating resistors, valves) are to be controlled from the machine microcontroller.
Generally, a bidirectional switch adapted to being controlled by a low-voltage signal is formed of a triac, the two power terminals of which are on the load supply line and the gate of which receives a low-voltage control signal.
A problem that arises with a triac is that it must be started for each halfwave of the supply voltage since it blocks (is non-conducting) when the current disappears between its two power terminals. Further, it is generally impossible to control the triac with a pulse signal at the mains frequency, in particular for small loads, since the hold and turn on currents of a triac are then of the same order of magnitude (some ten milliamperes) as the current in the load. Accordingly, in conventional systems, a D.C. signal is applied on the triac gate as long as it must be on.
A disadvantage is that this D.C. signal, generally provided by a microcontroller that draws its power from a low-voltage power supply, results in continuous power consumption.
Another disadvantage is that the supply circuit then dissipates a significant mount of power, since a voltage conversion from the A.C. supply voltage (for example, he mains) is most often used.
Finally, when one or several triacs are intended for being controlled by a microcontroller, this control by D.C. signals over the entire duration of the on periods monopolizes the microcontroller.
It would be desirable to have a high-voltage bidirectional switch that can be controlled by a low-voltage signal without generating significant power dissipation from the control circuit.
It would also be desirable to minimize the time of use of the control microcontroller, if present.
Thus, the present invention aims at providing a novel high-voltage bidirectional switch for controlling an A.C. load that can be controlled by low-voltage pulses, so that a first pulse can turn on the switch and that the switch remains on until a second pulse turns it off, whereby the switch is bistable.
The present invention also aims at providing a switch in which the power dissipation and the consumption is reduced or minimized.
To achieve these and other objects, the present invention provides a high-voltage bidirectional switch, including a high-voltage bidirectional switching element; and means for making the switching element bistable and controllable by, at most, two low-voltage pulse signals.
According to an embodiment of the present invention, the frequency of the pulse signals is small as compared to the frequency of the high A.C. voltage, the duration of the pulses being short as compared to the period of this high A.C. voltage.
According to an embodiment of the present invention, said means include two elementary switches and a capacitor to store a sufficient energy to restart the bidirectional switching element upon each halfwave of the high voltage.
According to an embodiment of the present invention, the capacitor is connected in series with a resistor and a first elementary switch between two terminals of the switch adapted for receiving the high A.C. voltage, the first elementary switch being intended for causing, by its turning on, the turning-on of the bidirectional switching element.
According to an embodiment of the present invention, a second elementary switch is series-connected with a second resistor, this series connection being connected in parallel with the series connection of the capacitor and of the first elementary switch, the second elementary switch being intended for, upon its turning on, discharging the capacitor to prevent an automatic restarting of the bidirectional switching element.
According to an embodiment of the present invention, the two elementary switches are controlled by a single control signal, each elementary switch being unidirectional and corresponding to a specific conduction direction.
According to an embodiment of the present invention, the single control signal is synchronized on the high A.C. voltage, the pulses appearing for a first sign of the halfwaves being intended for turning on the switch and the pulses appearing for a second halfwave sign being intended for turning it off.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.